1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses having a high optical performance throughout the entire zooming range suited to photographic cameras or video cameras. Still more particularly, it relates to wide-angle zoom lenses of the negative-lens-unit lead type having three lens units with a relatively high range or a zoom ratio of about 2.5 to 3, while shortening the total length of the entire lens system to a compact form.
2. Description of Related Art
It has been known to provide a method of improving the compact form of the lens system by assuring a shortening of the total length of the entire lens system and a reduction of the diameter of the outer lens barrel. Zoom lenses for photography which employ such a method are disclosed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 4-19706, Hei 5-127084 and Hei 7-77655.
Any of these zoom lenses is constructed with three lens units in total, comprising, in order from the object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of negative refractive power. The first and second lens units are made movable for varying the focal length of the entire lens system.
Of these zoom lenses, the ones disclosed in Japanese Laid-Open Patent applications Nos. Hei 4-19706 and Hei 5-127084 have their ranges in a zoom ratio of about 2 strong, and, therefore, cannot be said to be the high range zoom lens. With the lens configuration remaining unchanged, when the range is extended, a great increase of the total length of the entire lens system results, so that a compact zoom lens could not be obtained.
Meanwhile, the one disclosed in Japanese Laid-Open Patent Application No. Hei 7-77655 has as high a range as about 3, while still shortening the total length to a compact form. However, the rules of lens design are not satisfactory. Therefore, this zoom lens cannot be said to have good enough optical performance.